Since the invention of the pneumatic tire and its application to motor vehicles such as automobiles, trucks, and the like many schemes have been proposed to monitor the fluid pressure in the tires during operation of the vehicle. Such systems facilitate maintaining optimum fluid pressure in pneumatic tires which, in turn reduces the chances of catastrophic loss of vehicle control as well as extending tire life.
It will be apparent from a reading of the specification that the present invention may be advantageously utilized with pneumatic tires intended for many different applications. However, the invention is especially useful when applied to relatively high speed "on road" motor vehicles, and will be described in connection therewith.
A major problem in any tire pressure monitor is the transmission of fluid pressure information from the tire, i.e., a rotating body, to the operator i.e., a relatively fixed body. Most prior art approaches to this problem fall within three general catagories. The first approach is the direct reading of fluid pressure in a tire in which sealed, rotating fittings and electrical slip rings are employed in the interface between the wheel and vehicle body. The second approach is the transmission of fluid pressure information through an inductive coupling involving two transducers, one on the wheel and one on the vehicle body, which are in precise rotational alignment with one another. The third approach is the application of transmitters and receivers which use the atmosphere to bridge the interface between the wheel and body.
The first two approaches have several shortcomings. First, relatively expensive high precision components are needed in the slip rings and inductive coupling transducers which must remain in alignment at all times during operation of the vehicle. Additionally, both are prone to corrosion and mechanical wear at the point of interface as well as requiring relatively expensive body wiring from each wheel to a central location within the vehicle body.
Of the tire pressure monitors which have been commercially successful, most have followed the third approach, i.e., they have employed a transmitter affixed to each wheel which operates via a pressure switch to transmit a signal to a central receiver disposed within the vehicle body. These systems tend to be extremely expensive, however, and require a separate battery which must be periodically replaced. Additionally, transmitter-receiver approaches including interrogation type systems such as the transponder arrangement disclosed in U.S. Pat. No. 3,723,966 to Mueller et al. are contributors to RF pollution, the subject of current governmental regulatory study.
Also, most prior art systems employing any of the above described approaches include components such as transmitters, batteries and transducers which are mounted near the peripheral edge of the vehicle wheels, creating an imbalance condition which must be corrected by weights and the like. Additionally, due to the dynamic vibration involved in normal vehicle operation, the prior art systems have generally tended to deteriorate over a relatively short period of time, and those that did not suffer rapid deterioration were prohibitively costly for use with private passenger vehicles.
Finding a compromise solution for these problems has recently become even more urgent in light of government and industry interest in elimination of "spare" tires for cost and weight reasons and substituting "run flat" tires with a tire pressure monitor.